Over 90 million Americans (>40%) will seek medical attention for dizziness or some other balance disorder sometime in their life. An NIH working committee has reported that at least 2 million Americans experience chronic impairment due to dizziness or other balance disorders, causing medical expenses in excess of $1 billion per year. Some of these patients could benefit from a vestibular prosthesis that would be similar to the cochlear implant used to treat profound sensorineural hearing loss. Our work developing vestibular prosthetics is limited by shortcomings in our understanding of vestibular plasticity, since we cannot assess if responses to chronic vestibular stimulation provided by our prosthesis are restricted by device limitations or if the plastic limitations observed experimentally are fundamental and physiologic in nature. Our understanding of vestibular plasticity is limited because the traditional techniques (e.g., chemical or surgical labyrinthectomy, canal plugging) that are available to chronically manipulate the vestibular signals are not controllable and/or reversible. To address these clinical and scientific limitations, we propose to develop a new prosthetic device that can be used chronically to stimulate a semicircular canal. Our proposal is written in direct response to program announcement PA-04-006 ("Neurotechnology Research Development & Enhancement"). The new device that we propose to develop will enable scientific investigations that are not possible using existing techniques while also fundamentally advancing vestibular prostheses. Specifically, we propose to design, develop, and test a new dynamic, chronic, controllable, fluid-dynamic, canal stimulator. This device combines microcontroller circuitry, like that developed for our vestibular prosthesis, with a chronic mechanical actuator that pushes endolymph in an individual semicircular canal, which, in turn, deflects the cupula. This device will provide chronic dynamic control of endolymph movement, which is an essential part of normal rotation transduction, to an individual canal such that the gain, dynamics, and/or the apparent anatomic plane of the implanted canal can be controllably altered in non-human primates. The device will even be capable of providing canal stimulation in the absence of rotation. We propose to use this device to perform four hypothesis-driven scientific investigations. Specifically, we propose: 1) to measure high-frequency (>50 Hz) VOR responses, 2) to measure VOR adaptation evoked by changes in the vestibular signals as opposed to the standard approach of providing changes in the visual feedback that guides VOR motor learning, 3) to measure evoked potentials to evaluate efficacy and stability of the peripheral vestibular stimulation being provided, 4) to investigate the influence of canal stimulation on tilt perception, directly testing the hypothesis that canal signals influence tilt perception.